XMLHttpRequest Header : Decode Binary Data Upon Server Arrival - binary

Question

I am building a multipart/form-data httprequest, using Javascript's XMLHttpRequest object. Here are the headers:
Accept: */*
Accept-Encoding: gzip, deflate
Accept-Language: en-US,en;q=0.5
Content-Length: 38539
Content-Type: multipart/form-data; charset=UTF-8; boundary=CONTENT_BOUNDARY
Cookie: __utma=111872281.167592067.1396133699.1396133699.1396133699.1; PHPSESSID=ea7cfjgi8e29v1bhnrj580vbc2
DNT: 1
Host: localhost:90
Referer: localhost:90/my_secret_referer/no_peeking.php
User-Agent: Mozilla/5.0 (Windows NT 6.3; WOW64; rv:35.0) Gecko/20100101 Firefox/35.0
I send the request to a PHP document.
In the following snippet one can observe a part of the request body. It details the transmission of binary data in base64 representing an image file that has been selected.
--CONTENT_BOUNDARY
Content-Disposition: form-data; name="images[]"; filename="airplane.jpg"
Content-Type: image/jpeg
Content-Transfer-Encoding: base64
.../9j/4AAQSkZJRgABAQEBLAEsAAD/2wBDABkRExYTEBkWFBYcGxkeJT4pJSIiJUw3Oi0...
This body part cooperates with the entire request to create a successful AJAX request to my server. When the information arrives at the server, I decode the information and do what I may with it.
The segment of this process that I am unsatisfied with is the manual decoding of the binary data. One is always able to receive uploaded images in the server as complete, legible image files when using jQuery's $.ajax() or Malsup's jQuery Form. I have looked into the format of the two mentioned requests and have found that they too send binary information using a multipart/form-data request. The one difference that I have found which I have not intentionally made is that my request is furnished with a
charset: utf8;
specification inside of the Content-Type after I send it.
Can I, and if I can, how can I tell the server to decode the binary information it receives?